This invention relates to communication and, more particularly to cross connect systems.
There are two distinct types of network elements that are used in transport networks: Digital Cross-Connect Systems (DCS) and Add/drop Multiplexers (ADMs). With the advent of the SONET standard, these network elements have SONET capabilities and are called as SONET DCS and SONET ADM. It is to be understood that when the term SONET is used in this document, the corresponding international standard Synchronous Digital Hierarchy (SDH) is also included and it is interchangeable with SONET. FIG. 1 shows the architecture of a SONET DCS, and FIG. 2 shows the architecture of an ADM. As can be observed, their architectures are quite similar. The SONET DCS has a switch fabric 100 that is responsive to a controller 101, and line interface units 104, 114, and 124 that are coupled to the switch. Elements 101, 104, 114, and 124 operate under control of controller element 102. The ADM includes switch fabric 27, line interface units 2124, and controller 28 that communicates with the switch fabric and the interface units.
To elaborate, each interface unit in a SONET DCS contains one or more external system transmission interfaces that can handle different data rates. In SONET, the lowest rate is known as STS-1, and the higher rates that are typically employed are OC-3, OC-12, OC-48, and OC-192 (representing 3, 12, 48, and 192 times the data rate of OC-1). A system can start with one interface unit and grow with the addition of more interface units as needed. Each interface unit has one port at the output of the DCS and one or more ports connected to the cross connect fabric. The signals flowing through the ports that are connected to the cross connect fabric are sometimes referred to as xe2x80x9ctributaries.xe2x80x9d In a DCS, a tributary from any port can be connected to any other port through the cross connect fabric, under software controlled commands.
In addition to controller 101, the control structure for a SONET DCS typically comprises a central controller 102 that resides within the DCS, with connections to controllers within the interface unit and controller 101. The central controller also maintains communications interfaces for communicating to external control systems.
Functionally, the SONET DCS is used to multiplex and groom SONET payloads across the different SONET line rates. The DCS can connect traffic between rings, and manage complicated connections in the office by being a central point that connects SONET ADMs on different rings and that couples other equipment (such as an end office) to the network. The SONET DCS is not required to immediately reroute at the time of the failure. Normally, it reports failure events to a central system and waits to be told how to reconfigure in response to a failure.
It is also possible to reroute traffic through the SONET DCS in case of a failure autonomously, using a distributed communications protocol. However, implementation of distributed restoration using DCS is not so common. One reason is that it is not so simple to implement the distributed rerouting protocol in a DCS network, and the other reason is that it is not easy to make such rerouting as fast as it is done in a SONET ring.
While the SONET DCS and SONET ADM are architecturally quite similar, the DCS and the ADM are targeted for different applications, and correspondingly, their characteristics are quite different. To illustrate, the cross connect fabric of an ADM is extremely small (on the order of a dozen input ports and a dozen output ports) whereas the cross connect fabric of a DCS may be quite large (e.g., thousands of ports). The SONET ADM architecture is, effectively, a lower capacity version of the SONET DCS architecture.
More particularly, the SONET ADM comprises line interface units 21-24, drop interface units 25-26. In a typical four-fiber arrangement there is:
one (service) fiber pair (transmit and receive) that carries service traffic in the xe2x80x9ceastxe2x80x9d direction,
another fiber pair in the east direction whose primary function is to serve as a backup for the east direction traffic, should the service fiber fail (the backup or protection fiber in some applications is allowed to carry low priority traffic that may be preempted),
one (service) fiber pair that carries service traffic in the xe2x80x9cwestxe2x80x9d direction, and
another fiber pair in the west direction whose primary function is to serve as a backup for the west direction service traffic, should the service fiber fail, (the backup or protection fiber in some applications is allowed to carry low priority traffic that may be preempted.
One capability that is usually targeted to the SONET ADM is xe2x80x9cring functionalityxe2x80x9d. There are several types of SONET ring functionalities that are available in different SONET ADMsxe2x80x94Uni-directional path switched rings (UPSR), 2-fiber bidirectional path switched rings (BLSR), and 4-fiber BLSR. All of these rings comprise a sequence of SONET ADMs arranged in a closed loop. When a facility failure occurs, such an outside plant fiber being cut by construction equipment, the SONET ADMs react to the failure and reroute all of the traffic within 60-100 msec. This is done by the ADM whose one of two ports cannot handle traffic because of a failure condition in the link connected to that port applying that traffic to a spare fiber or to the fiber that carries traffic in the reverse direction in the ring. Although the arguments apply equally well to both 2-fiber and 4-fiber rings, our discussion here particularly adheres to 4-fiber case for simplicity of discussion. Because of this survivable architecture, the SONET ADM and the ring architecture are used in many critical network applications.
An ADM terminal in such an arrangement comprises four high-speed line interface units, one for each of the fiber pairs. Other than the drop-off traffic and the add-on traffic on the protection lines, under normal conditions the ADM cross connect sends all traffic from the east pair of line interface units to the west pair of line interface units. The drop interface units are interfaces that are employed for connecting to DCS elements, or to other elements (such as an end office). For an OC-48 ring typical drop interfaces are OC-3 and OC-12 and for an OC-192 ring typical drop interface units are OC-3, OC-12 and OC-48.
During the provisioning period of the ADM ring, messages are passed around the ring using the SONET Data Communications Channel (DCC) that is in-band within the SONET overhead. Some of these messages are used to configure the ring. At the time of a failure, bit-oriented messages are passed from node to node using the SONET K-byte protocol (henceforth, xe2x80x9cK-byte messagesxe2x80x9d). The ADM controller reads these messages from one side of the ring, processes the message and passes the message to the other side of the ring or back out in the reverse direction that the message came in. These messages are handled with low latency because they are designed to support the goal of restoration due to fiber failures in as little as 50 msec. As necessary (when a failure occurs), the ADMs cross connect reroutes the traffic to the backup fiber that sends traffic in the same direction as the failed fiber, or sends the traffic over the backup fiber that carries traffic in the opposite direction.
A typical network architecture using SONET DCS is shown in FIG. 3, where SONET rings 10, 11, and 12 are inter-coupled via SONET DCS 13. Traffic originating from a node in a ring (e.g., traffic that is placed onto ring 10 by ADM node 101 from drop 1011) and terminating at another node in the ring (e.g., ADM node 102) is routed merely over the ring. On the other hand, traffic between two nodes in two different rings is routed through the DCS, which forms an interconnection point between multiple rings.
For example, a signal between ADM nodes 114 and 121 is routed through ring 11, DCS 13 and ring 12.
As stated earlier, a conventional DCS primarily provides the function of grooming and routing traffic among the rings that are connected to the DCS and ports of the DCS that participate in inter-coupling different rings are connected to ADMs of those rings. Often, those ADMs are co-located with the DCS. Manufacturers have realized, of course, that it would be less expensive if the ADMs that are co-located as the DCSxe2x80x94such as ADMs 104, 124, and 115 in FIG. 3xe2x80x94were integrated with the DCS. One advantage of an integrated DCS is that the expensive optical interfaces for interconnecting the multiple ADMs and the DCS are eliminated. Another advantage of an integrated DCS is that it offers higher reliability. Still another advantage of an integrated DCS is that by combining the multiple network elements under the single control system it makes the operations and management functions simpler.
Other than the above-mentioned differences (and the advantages that accrue), the integrated DCS is essentially an aggregation of the DCS and the ADMs in a single equipment unit. In particular, each ADM-like port of the integrated DCS is coupled through line interface units to a small cross connect fabric. This cross connect fabric is connected to another line interface unit and to a drop interface unit that is internally connected to the main cross-connect fabric of the DCS. All ring switching functions of an ADM portion of the integrated DCS are performed by a controller that is associated with the ADM portion of the integrated DCS, including the passing of SONET K-byte messages between the line interface units. The DCS cross-connect fabric is used to connect low-speed interfaces to the high-speed ring interfaces and to interconnect the low speed interfaces of multiple rings. Thus, traffic that needs to go between two different rings does not need to be connected through external drop ports; rather the traffic is routed directly through the cross-connect fabric. The ability of connecting the low-speed tributaries from one ring to another without external physical low-speed interfaces in a DCS is a distinct feature of integrated rings compared to ADM-based rings interconnected by DCS.
The problem with ring arrangements that employ either the standard DCS/ADM connections, or the integrated DCSs, is the associated lack of flexibility in configuring the rings. In an ideal planning environment, a network operator would install raw transport capacity between nodes along fiber routes, and then use this capacity to create a ring structure, as needed and when needed, simply though software control from a central management system. That is not possible in the above-described arrangements. The ADMs (or the ADM portions of a DCS) have to be physically connected by fibers in a predetermined way. Once a fiber is connected to a particular ADM (or a particular port of the ADM portion of the DCS), continuation of the ring is fixed to that ADM (or to a specific port of the ADM portion of the DCS). The ring is constructed by going from one node to another (be it an ADM or an integrated DCS), each time making sure that the fiber going out of the node is connected to the other line interface of the ADM or the correct port of the ADM in the integrated DCS. This makes the ring structure fixed. If, at some point in time, it is necessary to change the ring configuration, it is not possible to do so without physically changing the fiber connections.
An improvement in the art is realized with a flexible integrated DCS that allows any port that is connected to a line interface unit within the integrated DCS to be coupled within the integrated DCSxe2x80x94pursuant to software controlled provisioningxe2x80x94to any other port that is connected to a line interface unit within the integrated DCS. This is accomplished by merging the cross connect fabrics of the ADM portions of the integrated DCS with the cross connect fabric of the DCS portion of the integrated DCS, by incorporating at least some of the switching controls of the ADMs in the ADM portion of the integrated DCS in the controller of the DCS, and by insuring that the SONET K-bytes can be passed by the controller of the flexible integrated DCS from any line interface unit to any other line interface unit. Further, the integration of the ring switch fabric and the cross-connect fabric gives rise to a need for inter-ring connection coordination when multiple rings are terminated in a DCS.